1. Field of the Invention
The present invention relates to a video signal recording and/or reproducing device which magnetically records and reproduces a video signal, that is, a video tape recorder.
2. Description of the Prior Art and the Problems
A television video signal magnetic-recording device, that is, a video tape recorder is well-known device for producing and preserving a video signal. Recently, a video tape recorder using magnetic recording technology has increased in acceptance as a device to record and reproduce video signal, such as obtained from a television program, or video camera or from a package soft.
Recently, requirements for high quality pictures have become higher, and for this purpose, a high vividness TV form, that is, a high density television (hereinafter called HDTV) has been proposed, and some development of a video tape recorder for such HDTV has being proposed. However, HDTV uses a wide band signal range which is about 5 times as wide as the usual television signal band. Thus, a very fast signal transmission rate is required.
If the video tape recorder for the HDTV were to be made based on the conventional video tape recorder system, it is necessary to make the diameter of a rotary drum carrying recording heads by about five times as big, i.e., about 30 cm. Such a big size drum is not acceptable from the practical viewpoint.
To realize the recording of such a wide band signal, remarkable developments have been made on high density recording since the recent development of magnetic recording technology, such that the recording density is improved every year. For example, by using a newly developed recording medium, such as metallic tapes and metallic heads, the recording density has been improved greatly. Thus, the amount of medium used for recording a certain amount of data has been reduced due to many new developments.
However, a number of problems still remain due to the following reasons:
(1) For television broadcasting, the wide band video signal should be transferred and recorded on a real time basis; PA0 (2) For accurately recording and reproducing the video signal in a wide band, it is necessary to increase the relative speed between the magnetic head and the recording medium; PA0 (3) The speed of a revolution of rotary drum mounted with the magnetic head is limited to a certain speed due to mechanical and structural limitations; and PA0 (4) The size of the rotary drum should preferably be smaller than a certain size for home use appliances, and from the viewpoint of mass-production.
The one segment recording (or non-segment recording) system in which one frame video signal is recorded on one track can not be realized. To overcome this problem, a two segment recording system has been proposed in which one frame is divided into two segments which are recorded on two tracks, one segment on each track. Alternately, a two channel recording system is also proposed in which the recording is effected two tracks at a time to prevent the deterioration of the transfer frequency reduction.
To determine which recording system should be used is very important because the track alignment determined by the selected recording system differs, and also consideration should be made on compatibility between different models and also on the effect of cross talk between neighboring tracks caused by the variation of head width.
FIG. 1 shows an example of a prior art track format employing a two segment recording system such that, for one field, four tracks are used, each of two heads scanned twice for recording two segments. According to this system, one picture frame is divided into upper and lower halves, each half being recorded on two tracks in parallel. With this system, the video signal can be reproduced to provide a proper image on the screen (FIG. 2a) at the ordinary reproducing speed, but at the high speed reproducing mode, the monitored video image on the screen will be doubled in upper and lower half portions of the screen (FIG. 2b) due to the separate recording of the video signal on the tape and also to the slant scanning of the magnetic head over the tape under the high speed reproducing mode.
In a magnetic recording and/or reproducing system, the difference in the azimuth angle between the recording head and the reproducing head gives rise to a reproduction loss shown by the following equation: ##EQU1## in which, L is an azimuth loss, .theta. is an azimuth difference angle, W is the track width and .lambda. is the recording wavelength.
This equation shows that, provided that the azimuth difference in the same track width is the same, the shorter the wavelength, that is, the higher the frequency, the larger the azimuth loss generated.
An azimuth recording system has been developed in which the neighboring two tracks are recorded and reproduced with recording and reproducing heads having different azimuth angles. When this system is employed, the cross talk between the neighboring track is reduced.
Referring to FIG. 3, an example of a prior art magnetic head arrangement provided on drum 1 is shown. The rotary drum 1 rotates in a direction of arrow F. There are four magnetic heads 3, 4, 5 and 6, which are also referred to as A-head, B-head, C-head and D-head, mounted on drum 1 for recording and reproducing the video signal. A-head and C-head are positioned on opposite sides of center axis 2 and similarly B-head and D-head are positioned on opposite sides of center axis 2.
FIG. 4 shows an example of positions of A-head 3 and B-head 4. A head 3 and B head 4 have gaps 7 and 8, respectively, which are slanted in opposite directions at an azimuth angle .theta. with respect to a direction perpendicular to the head running direction indicated by arrowed G. C-head 5 and D-head 6 are arranged in the same manner.
In the example shown in FIG. 3, if rotary drum 1 has a diameter .phi. of 76 mm, with a revolution speed of 3,600 rpm, a tape applying angle of 180.degree.+.alpha.=216.degree. (.alpha. is a margin angle for voice signal area and head switching area), a tape feeding speed of 79.8 mm/sec, and a track width of 24.8 .mu.m, a track pattern like the one shown in FIG. 1 will be formed on a magnetic tape 11.
That is, one frame is formed by 8 tracks, and picture information is divided into two segments for upper half and lower half of the screen image. The picture is also divided into a first field (even field) and a second field (odd field). Tracks 13 and 13' are used for the upper half of the first field, tracks 14 and 14' are used for the lower half of the first field, tracks 15 and 15' are used for the upper half of the second field, and tracks 16 and 16' are used for the lower half of the second field. In FIG. 1, N represents the divided order of the video signal. The voice signal is recorded on a PCM voice track 17.
The above track pattern, however, has the following problems. Since track 13 recorded by A-head 3 for the video signal of the upper half of the first field is located closely adjacent track 13' recorded by B-head 4 for the video signal of the upper half of the first field, a cross talk will be produced between tracks 13 and 13' in spite of the different azimuth angle, particularly for the low frequency range signal. Thus, the reproduced image will be deteriorated in C/N degradation of color signals, color blur or color irregularity. Although such a problem can be overcome by providing a space (guard band) between the tracks, this will not provide the high density recording.
In addition, when the video signal is recorded according to the pattern shown in FIG. 1, the reproduced picture on the screen will be proper (FIG. 2a) under the normal speed reproducing mode, but will be improper (FIG. 2b) under the double speed reproducing mode. Under a triple speed, or faster, reproducing mode, the picture will be utterly deformed.